Covalently attaching substituents to polysaccharides, for example hydroxyethyl, hydroxypropyl, and methyl, is well known as a way to modify various properties of the polysaccharide including solubility, viscosity, film formation, suspension of solids, and adhesiveness. Substituents which have carboxyl groups, for example carboxymethyl, can have additional properties including emulsion stabilization, binding of cationic species, crystal growth inhibition, and increasing the compatibility with other polymers. The carboxyl group can also be used to crosslink the ethersuccinylated polysaccharides either by formation of ester links or by ionic crosslinks between carboxyl groups. Such crosslinked ethersuccinylated polysaccharides can swell rapidly in water to form strong hydrogels. Substituents which are linked to the polysaccharides via an ether linkage, for example carboxymethyl, hydroxyethyl, hydroxypropyl, and methyl are advantageous since the ether linkage is stable under both acidic and basic pH conditions. Ethersuccinate is a substituent which contains both carboxyl groups and an ether linkage to the polysaccharide.
For some applications, a solution of ethersuccinylated polysaccharides at their native molecular weight has a much higher viscosity than desired. A typical reaction to reduce the molecular weight of polysaccharides is acid hydrolysis. In the case of starch this involves suspending the starch granules in a strong acid (hydrochloric or sulfuric acid) solution (pH approximately 0.75) at 40-55° C. for 12-14 hours. The granule suspension is then neutralized and filtered to remove the salt generated; the presence of salt can interfere with some applications. Over the course of the acid hydrolysis reaction a portion of the starch granule is solubilized and is removed in the filtrate thus reducing the yield of the conversion. The portion of starch solubilized becomes larger as the molecular weight target is lowered. Finally the reduced molecular weight starch granules are heated in water to provide a starch solution. Because of the low pH of the acid hydrolysis, it is not possible to include acid labile components, such as an enzyme to remove residual protein, during this step, thus another step is required for their inclusion.
In summary, existing processes for making reduced molecular weight substituted polysaccharides requires two steps. The first step is usually to react a native polysaccharide, for example a starch granule, with a derivatizing agent (i.e. ethylene oxide) to form a substituted polysaccharide (i.e., hydroxyethyl polysaccharide). A second step to reduce the molecular weight is then performed by reacting the substituted polysaccharide with a strong acid for long times, for example 12 or more hours, to finally obtain a reduced weight average molecular weight substituted polysaccharide. Alternatively, in the first step the native polysaccharide can be reduced in molecular weight by acid thinning the native polysaccharide to produce a reduced weight average molecular weight polysaccharide followed by a second step of reacting the reduced molecular weight polysaccharide with a derivatizing agent. In either case, strongly acidic, corrosive conditions are needed and salt is generated, which must be removed. Such acidic, corrosive conditions and salts are problematic and undesirable.
Accordingly, there is a need for a process which reduces the molecular weight of ethersuccinylated polysaccharides, for example ethersuccinylated starch, with high conversion, without the need to remove salt, without low pH (acidic) reaction conditions which are corrosive, while providing a solution of reduced molecular weight ethersuccinylated polysaccharide, optionally, in the presence of low pH sensitive components.